Unlimited Mode will take all achievements and technologies and group them by property. It is recommended to complete classic mode before turning on unlimited mode however there is an option in the settings menu to switch over early.

Pros

Gain as many achievements and technologies as you can afford.

Streamlines the purchase of technology by allowing a long press to "buy maximum."

Cons

All multipliers are averaged out (to between 3x and 4x depending on property), so the excitement of waiting for the x25 at level 1000 is missing.